security: eliminate panic sites in ember-core keyspace

[kacy]

summary

• replaced 6 sites of self.entries[key] direct hashmap indexing with safe .get()/.get_mut() + let-else patterns. direct indexing panics on missing keys; the safe patterns return early instead.
• replaced expect() calls in zrem, hdel, and hincrby with let-else returns
• hincrby and vadd type checks now use match on .get() instead of contains_key + direct index, eliminating a redundant lookup
• format_float now checks i64::MIN..=i64::MAX range before casting, preventing silent truncation on extreme float values (e.g. 1e18)

what was tested

• cargo test -p emberkv-core --features vector,protobuf — all 384 tests pass
• cargo clippy -p emberkv-core --features vector,protobuf -- -D warnings — clean
• cargo fmt — clean

design considerations

• the expect() calls in track_size (lines 389, 392) are retained because the closure requires &mut Entry — there's no meaningful fallback if the key doesn't exist, and the invariant is maintained by the immediately preceding insert_empty or ensure_collection_type call
• cleanup_after_remove now uses if let Some(entry) instead of direct index, silently skipping the memory adjustment if the key is somehow missing (defensive, not expected to trigger)